I can't speak for every person but the main argument isn't that there won't be apps not on the App Store, the argument is that apps won't be pulled from the App Store to be separate.
Touched on this in my last post, but there was always going to be smaller/independent developers that elect to just not be on the App Store and only be independent. The only major company who's given any indication they won't be on the App Store is Epic, and that's only because they are banned from doing so.
A major company like Spotify, or Meta, or Microsoft, or larger institutions like your bank or insurance company aren't going to force users to not use the App Store. We have empirical evidence from pretty much every platform that you have a much stronger reach when you're available in the main software distribution channel. That's not to say these companies won't also be available outside, but it seems pretty unrealistic they would leave the App Store. Delta, an app made by two people in their free time, isn't really representative of what a profit-driven multi-million/billion dollar company would do.